The RNA polymerases are not the rate limiting factors controlling RNA synthesis in eukaryotes. Inhibition of protein synthesis either by cycloheximide or amino acid starvation or by transfer of a thermo-sensitive yeast mutant to the non-permissive temperature for initiation of protein synthesis inhibits nuclear RNA polymerase (i.e., RNA synthesis in isolated nuclei) but not total RNA polymerase. This implies that inhibition of the protein synthetic machinery produces template restriction. As a consequence there is a diminution in the rate of utilization of ribonucleotide residues, mainly UTP and CTP. This in turn produces in some yeast strains, an inhibition in the rate of entrance of uracil and uridine into the UTP and CTP pools. The logical explanation of this phenomenon is that in these yeast strains the uridine kinase and uracil phosphoribosyl transferases are feedback inhibited by UTP and CTP. In the A364A and ts-187 mutants this inhibition does not occur, suggesting that these strains contain an uridine kinase and uracil phosphoribosyl transferase resistant to UTP and CTP inhibition. Therefore, this salvage pathway in RNA metabolism of nucleated cells has to be taken into consideration when there is a restriction in template transcription. Other important aspects of RNA metabolism that became apparent in our studies with yeast are the effect of temperature. Thus a shift-up in temperature causes the rate of lysine incorporation into the A364A and S288C strains to increase. However, the rate of uridine incorporation into the RNA is significantly, but transitorily, inhibited. The addition of small doses of cycloheximide suppresses this inhibition and in this condition the rate of uridine incorporation into RNA is much higher than the non-treated culture. This phenomenon is comparable to the addition of cycloheximide to amino acid-starved cells and suggests that upon increasing the temperature, a repressor-like component for RNA synthesis is synthesized. The drug either prevent its synthesis or its activity. BIBLIOGRAPHIC REFERENCES: K.J. Gross and A.O. Pogo (1976). Control of Ribonucleic Acid Synthesis in Eukaryotes II. The Effect of Protein Synthesis on the Activities of Nuclear and Total DNA-Dependent RNA Polymerase in Yeast. Biochemistry, in press.